1. Field of the Invention
This invention relates to gimbaled imaging systems with an off-gimbal electro-optic source or detector, and more particularly to ball joint gimbal imaging systems with an off-gimbal directional electro-optic component in which on-gimbal optics reimage the front optical aperture to a smaller back optical aperture that moves with the rotation of the inner ball.
2. Description of the Related Art
An EO imaging system includes a fixed directional EO component such as a detector or laser source and a gimbal for controlling a pointing axis to move a field-of-view (FOV) of an optical system in two axes through a field of regard (FOR), In certain configurations, the EO component is mounted “on-gimbal” and rotates with the FOV. In other configurations, the EO component is mounted “off-gimbal” and remains fixed as the FOV rotates. In both cases, a front optical aperture is imaged into (or imaged from, for sources) the EO component. In some on-gimbal cases, the front aperture is first reimaged (or relayed) into to a smaller back optical aperture and through a pupil stop to limit stray light, and then imaged into the EO component. In “off-gimbal” configurations, a back aperture is formed (reimaged from the front aperture) to reduce the beam size so that light may be relayed off gimbal via coudé paths, through the gimbal axes, and then imaged into a off-gimbal EO component. These types of EO system may be used, for example, on aircraft or various types of munitions e.g. missiles, rockets, artillery shells, etc.
The classic method to control two-axis pointing of a pointing axis is to control (and measure) rotation separately in each of two axes (Roll/Nod, or Az/El which is also known as Yaw/Pitch) through a nested gimbal arrangement in which a first gimbal is mounted on a second gimbal. The axes of rotation of the first and second gimbals are perpendicular to each other such that each axis of the nested gimbal controls one axis of rotation. Gimbal drive motors are configured to mechanically rotate each gimbal about its axis. With a two axis system, the third rotational axis is kinematically constrained by the position of the first two gimbals. For example, a particular Az, El or yaw/pitch angle pair rigidly specifies a unique roll angle. Roll cannot be independently controlled without adding a third gimbal or some equivalent.
When the EO component is off gimbal, reimaging optics are implemented via coudé paths, where nominally collimated light is relayed through the rotation axis of each nested gimbal. In some embodiments, the optical coudé path may include at least two mirrors to provide a bi-directional communication path through an azimuth axis and an elevation axis of the gimbaled payload.
Another method to control two-axis pointing of a pointing axis is via a ball joint gimbal, in which an inner ball is captured within a socket. The inner ball is free to rotate about combinations of three orthogonal axes, except as constrained by the motor and control system. A motor is configured to apply forces to rotate the inner ball. Different motor configurations include ultrasonic motors, mechanical tendons and linear electro-magnetic permanent magnet motors. In some cases, notably tendon drives, the ball position is determined directly from the actuation system (rotary encoders on the tendon drives). In others, it is determined via separate gimbaled position readouts, attached about the ball. In still others, non-contact means are used to directly measure the position of the ball surface relative to a sensor.
In the prior art, ball gimbals do not allow for a conventional coudé path. On-gimbal configurations of the EO component place the component within the inner gimbal, so this is not an issue. Off-gimbal configurations that position the EO component off-gimbal are problematic. Unlike the classic nested gimbal configuration, the ball joint gimbal does not have fixed axes of rotation, so there is nowhere to place a coudé path to relay the optical aperture on the inner ball to the off-gimbal EO component.
Other systems use a ball gimbal which, unlike the systems here, does not contain imaging or reimaging (relay) optics, but which simply moves a flat mirror. These are not gimbaled systems, but rather, are strap-down systems with a gimbaled fold mirror. This moving mirror is placed within the optical path to redirect the optical rays, so that it serves as a beam steering element rather than as a directed optical subsystem. U.S. Pat. Nos. 6,326,759 and 7,032,469 are directed to a ball joint gimbal system that provides for a precise line-of-sight stabilization of a gimbaled mirror that rides on a ball and its associated support structure. The mirror is positioned by four braided lines driven by corresponding servo motors. A target image is formed on a fixed imager by reflecting the light off the gimbaled mirror along an optical path to a fixed mirror and then along a light path through focus optics to the fixed imager. The image of the target is stabilized in inertial space using the electrical signals from missile body mounted rate gyros to cause the gimbaled mirror to move the correct amount to compensate for the motion of the missile.